1. Field of the Invention
The invention relates to a member for the formation of at least one electrode and/or one sensor, having a carrier and a metal oxide layer which is present thereon and consists of at least one oxide of at least one metal belonging to the fifth or sixth period or to one of the subgroups 5b, 6b, 7b or 8 of the Periodic Table of chemical elements and/or of zirconium.
The member may be provided, for example, to form, in contact with an electrically conducting liquid--i.e. an electrolyte--an electrode which acts as a proton donor and/or proton acceptor. The member can be used, for example, for the measurement of the pH value. As will be explained, the member may, however, instead or in addition form or have an electrode and/or sensor for other purposes.
2. Description of the Prior Art
The unexamined Japanese Patent Application 4-050 648 discloses a biochemical sensor having a sapphire lamella which carries a p-doped silicon film. The silicon film is covered with an insulating film on its side facing away from the sapphire lamella. Light emitting diodes by means of which a region of the silicon film can be activated by illumination are also present. This sensor thus has no layer consisting of one of the metal oxides stated further above and has the disadvantage that its silicon film has to be illuminated for the measurement by means of the light emitting diodes.
The unexamined Japanese Patent Application 58-151 396 discloses that an oxide of bismuth and of one of the elements silicon, germanium, gallium or titanium, which oxide has the structure of a gamma-bismuth single crystal, can be heated in order to increase the electrical resistance and then used for the formation of an electro-optical sensor. However, the oxide contains none of the above-mentioned metal oxides present in the member according to the invention.
U.S. Pat. No. 3,726,777 describes a member which has an iridium oxide layer and is used as an electrode for the measurement of the pH value of blood and other liquids. According to this publication, an iridium wire is preferably dipped into a potassium hydroxide solution or sodium hydroxide solution and then heated to about 800.degree. C. in an oxygen environment for the preparation of the oxide layer.
The publication "Preparation of Iridium Oxide and its Application in Sensor-Actuator Systems", W. Olthuis, J. C. van Kerkhof, P. Bergveld, M. Bos, W. E. van der Linden, Sensors and Actuators B, 4, 1991, pages 151-156, Elsevier Sequoia, discloses a member which is provided with an iridium oxide layer and is used as an electrode for donating and accepting hydrogen ions and for coulometric titration. According to the publication, for the production of an electrode, a carrier having a silicon lamella and an iridium film arranged thereon is first produced. This is then oxidized electrochemically in a liquid containing sulfuric acid.
The publication by Olthuis et al. states that the immersion of electrodes in electrolytes for a total period of 120 hours causes a reduction in the proton exchange rate by about 18%, and that two successively performed concentration measurements of hydroxyl ions and hydrogen ions at constant concentrations give deviations of the measured values which are within 2% and 1%, respectively. According to our own investigations of electrodes which were produced by methods of the type described in the cited U.S. Pat. No. 3,726,777 or the cited publication by Olthuis et al., such electrodes do in fact have the disadvantage that their proton acceptance rates and proton donation rates and sensitivities of measurement vary considerably in the course of time. Such changes in the properties of the known electrodes as a function of time can present problems not only in measurements of the pH value but also in other applications of an electrode where the electrode serves as a sensor--i.e. as a measuring element--and/or as a control element and/or as an excitation element.
According to U.S. Pat. No. 3,726,777, the iridium oxide layers known from this publication have a bluish black color. According to our own studies and investigations, the production of oxide layers by methods of the type described in the cited publication by Olthuis et al. also leads to dark, substantially opaque oxide layers. Members having iridium oxide layers of the types known from the cited publications would therefore also be unsuitable as a measuring element for optical methods of measurement in which the oxide layer must be transparent to light.